In stainless steels, dissolved carbon and nitrogen increase the hardness of the martensite, the yield point of the austenite, and effect a stabilization of the austenitic phase. Whereas the addition of carbon adversely affects the resistance of stainless steel to corrosion from moisture, nitrogen effects an enhancement of this property. Standing in the way of the utilization of this favorable effect of nitrogen is, in contrast to carbon, its considerably lower solubility in the molten steel at normal pressure. For this reason, pressure and powder metallurgy techniques are used these days in order to produce stainless steels having a nitrogen content between 0.3 and 3% by weight. However, these techniques are far more expensive than an open steel smelting process.
German Patent 40 33 706 describes casehardening with nitrogen, whereby after the nitrogen enrichment of a martensitic stainless steel by hardening, a hard, martensitic surface layer is produced over a ductile core. This process is used for treating rust proof ballbearings, transmission parts and tools, as well as for rust proof pump parts and valves in particle-laden fluids. In all of these cases, the concern is with maximum resistance to pressure and hardness of the surface layer, which however is accompanied by significant brittleness.